In the field of polarization-based light valve projectors, reflective light valves are presently employed in projection devices since they allow a single set of beam-dividing elements to be used twice; first for color separation, and then for recombination. The use of one or more dichroic coatings in prior art double-pass optical arrangements gives rise to contrast problems however because the three-dimensional beam geometry causes polarization mixing. Both polarizations must be allowed to reflect or transmit from such double-pass coatings; this rules out pre- or post-filtering to correct the polarization mixing.
Even if double-pass color prisms are not employed, reflective light valves require that at least one coating be used twice, namely the polarizing beam splitter (PBS). Some loss in contrast arises even from this one coating, particularly as the numerical aperture (NA) increases. The term "numerical aperture" is used herein to denote the sine of the illuminating solid cone angle or the square root of the solid angle/.pi..
Even though this contrast problem exists and has been known for several years, there are no known solutions in the prior art for solving this problem which can be used in all optical systems and/or configurations. One known solution to this problem which is specific for a single light valve polarization beam splitter system is described in European Patent EP 0 389 240 A2 to Matsushita, Electric Industrial Co., Ltd. ("EPA '240"). Specifically, EPA '240 discloses a quaterwave plate that completely corrects a compound angle depolarization from a polarizing beam splitter, if a light valve behaves like a mirror when switched to the black-state.
Although the prism system disclosed in EPA '240 is reportedly useful in improving contrast in a single light valve polarization beam splitter system, it can not be employed in all optical configurations, or with most polarization-modulating light valves. Instead, it is specific for use in a single light valve polarization beam splitting system such as described in the disclosure of EPA '240. Although the prism system disclosed in EPA '240 has limited use, it is presently being used in many projection display devices.
In view of the above-mentioned drawbacks in the prior art, there is a continued need to develop new and improved optical elements, i.e. beam dividing elements, which can be employed to improve contrast imaging in any optical configuration, or with any polarization-modulating light valve. Such an optical element is particularly needed with liquid crystal light valves.